This invention relates to acoustic surface wave devices. More particularly, it is concerned with acoustic surface wave devices employed as filters.
Acoustic surface wave devices employing piezoelectric materials having suitable properties for propagating surface waves and having transducers for launching and receiving acoustic surface waves in the material are well known. Typically, the transducers are arrays of interleaved conductive electrodes deposited on a substrate of the material. In response to electrical signals an input or transmitting transducer launches acoustic surface waves along a predetermined path on the surface of the substrate. An output or receiving transducer detects the acoustic waves and generates electrical signals in response thereto. Typically, acoustic surface wave devices have been employed as delay lines and as filters. Because of the frequency response which can be obtained in an acoustic surface wave device by suitably designing the configuration of the transducer electrodes, particularly desirable bandpass characteristics can be achieved for use of the device as a filter.
In the development of acoustic surface wave devices for use as filters various problems have been encountered. Several secondary effects are present which tend to degrade the performance of the device. Various techniques have been employed to compensate for or avoid certain of these secondary effects. One significant problem of acoustic surface wave devices is the presence of "triple transit signals" which result from the interaction between the input and output transducers. In response to the receipt of acoustic energy from the input transducer, the output transducer causes a fraction of the energy to be directed back toward the input transducer. The input transducer re-transmits a portion of this energy to the output transducer. Thus, a greatly reduced but nevertheless noticeable echo signal is received by the output transducer. This signal which transits the distance between the input and output transducer three times distorts the electrical signal produced by the output transducer.
Various techniques have been employed to eliminate or reduce the effects of these triple transit signals. Reflections of acoustic energy from the edges of the electrodes of the transducers can be suppressed by using electrodes having two elements of one-eighth wavelength in width and separation in place of single element electrodes of one-quarter wavelength in width and separation. Other techniques have been devised in attempts to reduce the effects due to the regenerative action of the received energy with the transducers.
One procedure which may be employed to suppress triple transit signals caused by regeneration is to increase insertion loss. Although increasing insertion loss reduces the signal, the additional suppression of the triple transit signal is twice that of the additional insertion loss. A common technique for increasing insertion loss is the mismatching of the electrical impedance of the device. This technique, however, may cause distortion of the signal and is not effective to the reflections caused by the electrode mass loading and impedance discontinuity. In addition, the electrical impedance mismatching must be achieved externally of the acoustic surface wave device by adjusting the values of the components connected thereto.